I,m confused as to what material ,to use for my 6" hearth base that my fire brick will set on . My understanding or misunderstanding is that this slab needs to withstand, absorb and retain the heat from the oven.my hearth will be 10" thick . 6" of a poured base and 4" of fire brick.I want the full 10" to absorb and retian the heat.I will have an insulated slab under the hearth.Please help .